FIG. 17 illustrates an arrangement of a conventional motor speed control device.
In the figure, 51 is a polyphase brushless motor, 52 is a rotation detecting sensor such as an MR sensor for detecting a magnetic pattern formed on a motor rotating section, 53 is a waveform shaping circuit for amplifying an output signal supplied from the rotation detecting sensor 52 so as to output a rotational pulse signal having pulses proportional to the motor rotation in number, 54 is a cycle computing unit for outputting cycle information of the pulse signal, 55 is an adder, 56 is an amplifier for amplifying an error signal supplied from the adder and performing phase compensation so as to output a speed command value, 57 is a motor driving circuit for switching power supply to each coil in response to signals from sensors such as a Hall element (not shown) for detecting a rotor position, and driving the motor in accordance with the speed command value. The motor speed control device is composed of these components.
Feedback control is performed with respect to the motor rotational speed as follows: cycle information as information on the rotation of the motor is compared with a target value, and power supply to motor coils is switched based on an error found by the comparison, so that the cycle information comes to coincide with the target value.
One factor causing fluctuation of the motor rotational speed of the polyphase brushless motor is so-called a motor torque ripple, which is a phenomenon as follows: a motor torque cyclically ripples at every switching of excitation of the motor coils, thereby varying the motor rotational speed. The motor torque ripple is caused due to, for example, inconsistancy in exciting current at switching of motor excitation, or heterogeneity in magnetic flux density at a rotor magnetic pole switching point. Usually, one fluctuation component, the number of times of whose appearance per one rotation of the motor is a least common multiple of the number of the coils and the number of the rotor magnetic poles, occurs more greatly than the others. In addition, another fluctuation component, the number of times of whose appearance per one rotation is an integral multiple of the number of the rotor magnetic poles, also occurs. For example, in the case of a three-phase brushless motor with 6 driving coils and 8 driving magnetic poles, a speed fluctuation component at a great degree appears 24 times per one rotation of the motor. Such speed fluctuation components due to the motor torque ripple cannot be fully eliminated by the conventional motor speed control device, and therefore, motor control with high precision has been difficult.
To eliminate the speed fluctuation components due to the motor torque ripple, the applicant of the present application has filed a patent application of an invention which has been disclosed in the Japanese Publication for Laid-Open Patent Application No. 6-54571/1994 (Tokukaihei 6-54571). The following description will briefly explain the arrangement of the invention disclosed in Tokukaihei 6-54571. An amplitude and a phase of a motor torque ripple of a motor to be controlled are preliminarily measured, and an amplitude of a sinusoidal wave data (gain set value) and a phase (start address) are set as corrective data, in accordance with the measured amplitude and phase of the motor torque ripple. Then, rotational position information (FG address) is detected based on position information (PG) and rotational information (FG) of the motor, and the corrective data is recalled in accordance with the rotational position information and are added to the motor command value. Since the corrective data is set so as to cancel the motor torque ripple, the motor torque ripple are corrected and the motor rotation control can be realized with high precision.
According to the invention disclosed in Tokukaihei 6-54571, the correction is basically possible with respect to any component of a motor torque ripple as long as the number of times of appearance of the motor torque ripple is an integral multiple of the number of rotations of the motor. Besides, the correction is performed by a simple process of recalling the corrective data, irrelevant to an amplitude and phase of the motor torque ripple. Therefore, it is a very effective method wherein the motor torque ripple is a corrected with simple processes.
However, the invention disclosed in Tokukaihei 6-54571 has following drawbacks.
First of all, though it has the aforementioned advantage in that the correction is possible with respect to any component of a motor torque ripple as long as the number of times of appearance of the motor torque ripple is an integral multiple of the number of rotations of the motor, it has a drawback in that an operation for measuring position information (phase and amplitude) of motor torque ripples is necessitated as to each motor, thereby making the correcting operation troublesome. Besides, since corrective data for one rotation of the motor has to be stored in the case where the number of pulses of the position information PG per one rotation of the motor is one, a corrective data amount and memory capacity expand.
Secondly, since the rotational position information (FG address) is set to different values throughout one rotation of the motor so that the corrective data is recalled, position information PG for exclusive use for detection of the rotational position information is required.
Thirdly, since amplitude information of the motor torque ripple is a value which has been preliminarily measured, effects may decrease when the amplitude varies at every drive of the motor, or when the amplitude varies during rotation of the motor.